1. Field of the Invention
The present invention relates to sound field correction technique for correcting the influence on frequency characteristics caused by a standing wave in a room.
2. Description of the Related Art
In the case where sound is emitted from a sound source such as a loudspeaker in a room of a house or the like, since there is reflected sound from surfaces such as a wall, a ceiling, and a floor of the room in addition to direct sound that arrives at spots in the room over the shortest distance, these sound waves become superimposed on each other. At this time, for example, a standing wave is generated, and bass resonance called booming occurs between the surfaces facing each other in parallel in the case of a frequency at which the distance between such surfaces is an integral multiple of the half-wave length of the sound wave.
In such a case, booming is suppressed with a parametric equalizer, or acoustic characteristics are measured in advance at a listening position using a microphone, and correction is performed based on the inverse characteristics thereof. Furthermore, in addition to such technology, technology utilizing direction information of reflected sound is also disclosed (for example, see Japanese Patent Laid-Open No. 5-83786).
If frequency characteristics of a listening room or the like are measured, the characteristics as shown in FIG. 2 can be obtained, for example. A standing wave is generated in a peak portion in which the sound pressure level is increasing and in a dip portion in which the sound pressure level is decreasing. The standing wave portions have a frequency at which the sound output from a loudspeaker or the like resonates with respect to the size of the room, and have not only a greater level of fluctuation, but also a greater change in a time direction relative to other frequency portions.
The influence due to a standing wave will now be described with reference to FIG. 3. In FIG. 3, a signal 33 is a signal having a frequency of a dip portion. A signal 32 is a signal having a frequency of a flat portion in terms of frequency characteristics, and a signal 31 is a signal when the signal 32 is emitted in bursts. The sound pressure level of the signal 32 corresponding to the flat portion steeply drops following the fall of the burst-like signal 31.
The signal 33 corresponding to the dip portion starts rising in a normal manner in the leading edge portion thereof in the state where there is no reflected wave. However, since the signal 33 corresponding to the dip portion has a frequency at which a standing wave is generated, upon the start of interference with a reflected wave, the level thereof becomes lower during the occurrence of the burst signal, due to interference of a direct wave and a reflected wave. Furthermore, since the signal 33 is in the resonance state at a standing wave frequency, although the original burst signal has fallen, the signal is observed having a higher level than that while sound is produced. This is because the component of the direct wave is lost along with the end of the burst signal, and thus only the component of the reflected wave that has increased due to resonance remains, which allows a signal having a higher level than that during the sound production period to remain for a long time in spite of the end of sound wave output. For this reason, the signal component of the dip portion has a lower level while sound is originally produced, and has a higher volume at the time when sound should not be produced, which causes a problem concerning auditory sensation.
Further, a frequency of the standing wave peak portion also has a problem that loud reverberation remains for a long time, for instance. In the case of general booming correction, a technique is employed in which a frequency component corresponding to the peak portion of a standing wave is always attenuated by a fixed amount using a parametric equalizer or the like. However, if this technique is applied to the dip portion, negative effects are caused, one example of which is that a portion where the original sound that has already been decreased due to interference is further attenuated, and thus the sound of that portion can hardly be heard.